Thin film, ferrimagnetic materials such as rare earth-transition amorphous alloys of terbium iron cobalt (TbFeCo), gadolinium terbium cobalt (GdTbCo) and gadolinium terbium iron cobalt (GdTbFeCo) have been known as high-density, magneto-optic recording media. Magnetic domains on the order of one micrometer in size can be recorded in the magneto-optic material. These ferrimagnetic materials have high coercivity at room temperature and low coercivity at high temperatures. The recording medium, preferably in a coated disk form, can be magnetized in a particular direction perpendicular to the surface by heating the disk in the presence of an external magnetic field, and then permitting the disk to cool or by applying a saturating magnetic field. Data can thereafter be stored on the disk by heating a small spot (preferably by laser energy) in the presence of an external magnetic field of the desired magnetic polarity. The heated area is magnetized in the direction of the external magnetic field when the area cools and returns to the high coercivity state at room temperature. Data on the disk is "read" by noting the effect on polarized light reflected off the disk surface.
Magneto-optic recording systems operating without external magnetic bias are also known. Compensation point systems operating without magnetic bias developed by some of the inventors hereof use a magneto-optic medium with a compensation temperature a few tens of degrees C. above room temperature and preferably at least 50.degree. C. below the Curie point temperature. A stable magnetic domain can be recorded on such a medium by heating a local area above the compensation point using a laser beam and by relying upon the magnetic self bias to invert the magnetic polarity within the domain. The domain can subsequently be erased by applying a lower energy level laser pulse to create a domain within the previously recorded domain. If the internal domain is created at the correct laser pulse energy level, the internal domain wall tends to expand as the surrounding domain wall tends to contract until the domain walls annihilate one another, thereby erasing the previously recorded domain.
Curie point systems operating without external magnetic bias are also known, but operate on a significantly different principle. The magneto-optic medium must have a relatively low Curie point, preferably in the range of 80.degree. C. to 180.degree. C. If an area is heated above the Curie temperature, the heated area loses its magnetization and, upon cooling, forms a stable domain of reverse magnetic polarity at approximately one-half the radius of the area heated above the Curie point. To erase a previously recorded domain, the previously recorded domain area is heated above the Curie point and the magnetization of the heated area disappears. If the erase pulse is small and just sufficient to heat the area of the prior domain above the Curie point, the domain of reverse magnetic polarity that tends to form upon cooling is unstable and therefore collapses upon cooling.
Biasless systems operating without external magnetic bias do not suffer from the time lag required with changing magnetic fields, and therefore such biasless systems are capable of extremely high operating speeds. Such biasless systems also avoid the need for bulky magnetic components close to the recording medium.
In the previously known systems the recorded domains were thought to stabilize within cylindrical domain walls corresponding to the lowest energy configurations. See, for example, "The Theory of Cylindrical Magnetic Domains" by A. A. Thiele from the Bell System Technical Journal Vol. 48, No. 10, December, 1969. The mobility of magnetic domains resulted in the development of magnetic "bubble" memories. In creating a magnetic domain, if the applied thermal energy is too low, the domain will collapse. If the applied thermal energy exceeds a minimum threshold level (and is not too large), a domain is established which expands. There are several counteracting forces affecting the dynamics of domain formation which tend to form the domain at a minimum energy level configuration.